1. Field of the Invention
The present invention relates to a system for transmitting information signals.
2. Related Background Art
In recent years there have been significant progresses in the definition of images, as exemplified by the television image from the NTSC system to the high-vision system. In the recording of such high-definition image on a recording medium such as a tape or a disk, or in the transmission of such image to a distant location by a satellite, an optical fiber or a cable, digital transmission is superior to analog transmission in consideration of factors deteriorating the image quality such as the S/N ratio of signal, jitter generation, etc.
Such digital transmission provides an advantage in that the image quality deterioration is not generated in record-reproducing systems employing tape or disk even after repeated dubbings, but may lead to a major social issue, because it facilitates frequent illegal copyings or dubbings. Also in a charged communication in a communication or broadcasting system utilizing satellite, optical fiber or cable, there may result illegal reception of information by unauthorized persons.
For this reason, in the conventional digital signal transmission, particularly in the data transmission particularly for computers, there has been employed a method of applying predetermined ciphering to all the data to be transmitted, and deciphering the data at the receiving side with cipher key data corresponding to said predetermined ciphering.
In the following there will be explained conventional examples of such ciphering, with reference to FIGS. 1 and 2.
FIG. 1 is a flow chart of a ciphering method of the U.S. data encryption standard (hereinafter called DES) disclosed in the FIPS gazette 46 of Jan. 15, 1977, and FIG. 2 is a flow chart of a method for forming the ciphering function shown in FIG. 1.
Such conventional data ciphering algorithm shown in FIGS. 1 and 2 is already published as "Data Encryption Standard" as mentioned above.
This data encryption standard will be explained in more detail, with reference to FIGS. 1 and 2.
Said data encryption standard is a block cipher system for binary data consisting of "0" and "1". In this system, the ciphering is achieved by dividing the binary data into paired blocks of 32 bits each, and repeating transpositions and replacements to each block. The cipher key data have 64 bits, of which 56 bits are effective while the remaining 8 bits are error detection bits, and said key controls the replacement each time. FIG. 1 shows the process of ciphering in this system, and FIG. 2 shows a function fK(R) used as the core of ciphering.
Referring to FIG. 1, the binary data of 64 bits are at first subjected to a transposition, which is a predetermined process irrespective of the cipher key data. Then the binary data of 64 bits are divided into a left half L.sub.0 and a right half R.sub.0, which are then subjected to the following operations for 16 times: EQU L.sub.n =R.sub.n-1 EQU R.sub.n =L.sub.n-1 fK.sub.n (R.sub.n-1) (1)
wherein "+" indicates a sum of mod2 for each bit, while L.sub.n and R.sub.n are respectively the binary data of 32 bits of the left half and those of the right half after an n-th operation.
K.sub.n is formed from the cipher key data, as shown in the right half of FIG. 1, wherein s.sub.1, . . . , s.sub.16 are "1" or "2". The contractive transposition consists of a transposition with certain input bits excluded. For example 8 bits are excluded from the input 56 bits, so that the output becomes 48 bits. Said contractive transposition is an irreversible conversion, in which the input cannot be completely restored from the output, so that the estimation of the cipher key data is rendered more difficult.
The function fK.sub.n (R.sub.n) shown in FIG. 1 will be explained further in the following, with reference to FIG. 2.
For forming the function fK.sub.n (R.sub.n), at first R.sub.n (binary data of 32 bits of the right half) are subjected to an expansive transposition, which is a transposition with certain input bits overlapped. In this case, 16 bits out of the 32 input bits appear in overlapping manner in the output. Then K.sub.n, formed from the cipher key data, is added to said output, bit by bit in mod2. The output of 48 bits thus obtained is divided into 8 small blocks of 6 bits each, and each 6 bits are converted into 4 bits by S.sub.1, S.sub.2, . . . , S.sub.8. This operation can be regarded as a kind of replacement, taking 6 bits as a character, but this conversion is irreversible because the output is compressed to 4 bits. Consequently the function fK.sub.n (R.sub.n) is generally an irreversible function, but this does not mean that the conversions of equations (1) are irreversible. In fact said equations (1) can be transformed as: ##EQU1## so that L.sub.n-1, R.sub.n-1 can be calculated from L.sub.n, R.sub.n.
L.sub.16 and R.sub.16, obtained by repeating the operation of the equations (1) 16 times, are finally subjected to a transposition to complete the ciphering.
The deciphering is conducted by a process substantially inverse to the ciphering process, namely a process proceeding from bottom to top in FIG. 1. At first there is conducted a transposition inverse to the final transposition in ciphering process. Then R.sub.n-1, L.sub.n-1 are determined according to the equations (2), and R.sub.0, L.sub.0 obtained in this manner are subjected to a transposition inverse to the initial transposition in the ciphering process to obtain the initial 64 bits.
The only way so far known for breaking the data enciphered by this system is to investigate the cipher key data one by one. If a key requires 1 microsecond for investigation, the entire key of 2.sup.56 combinations requires 2283 years, or at least several hundred years with good luck, so that this system is practically unbreakable.
In case of digital transmission of high-definition television signal, such as the high-vision signal, with ciphering, if the analog image signal is simply A/D converted for transmission, the sampling rate has to be at least 60 MHz, according to the sampling theory, in order to secure an image signal bandwidth for example 30 MHz for preventing deterioration in image quality. For a sampling frequency of 74.25 MHz and an A/D conversion of 8-bit digitization per sample, the signal transfer rate becomes 74.25 (MHz).times.8 (bit)=594 Mbit/s. Even if the data are compressed to 1/5 for reducing the amount of transmitted information, the transfer rate becomes as high as about 120 Mbit/s. The enciphering of such enormous amount of information requires a high-speed process in the ciphering circuit, and is extremely difficult in the magnitude of hardware and in the increased cost.